There is known a plurality of animal growth promoting agents represented by the following general formula: ##STR1## wherein R.sub.1 and R.sub.2 are substituents such as hydrogen, hydroxy or lower alkoxy; R.sub.3 is a substituent such as hydrogen or lower alkyl; R.sub.4 is a substituent such as lower alkyl, lower alkanoyl, aryl, or aralkyl; A is a bridging group such as --CH.dbd.CH-- or --CH.sub.2 --CH.sub.2 --; Z is a bridging group such as &gt;C.dbd.O, &gt;CHOH or &gt;CH.sub.2 ; and when R.sub.3 is lower alkyl, said compounds are the racemic mixtures and the optical isomers thereof. Among the optically active isomers the (3S) isomers are preferred, since they appear to be biologically more active than the corresponding (3R) isomers.